First, allowing any battery to discharge to 50% is not good. I would discourage allowing it to drop to more than 25%. This is a valid point for all types of batteries, and allowing it to discharge to that level is one of the worst things that you can do to a battery. But actually, the WORST thing you can do to a battery is to overcharge it.
Hooking up to "shore power" and charging your battery via the converter's built in charger for a few days now and then is fine, but even then I would keep an eye on your water levels every few weeks to make sure the water in the cells hasn't dropped below the level of the top of the lead plates. You don't want the tops of the lead plates to be dry, because this will destroy your battery through a process called "sulfation," which isn't a good thing to have happen, and will shorten the life of your battery.
Now, another story, in your question you make reference to a battery tender...I'm not sure if your are using an actual Battery Tendertm (as in the brand name,) but if you are, please be advised that it is a trickle charger, and these little units, (although they are low current trickle chargers,) are non-the-less continual chargers. In other words, they don't regulate or alter their charging currents to reflect the charge level that the battery is at. They will continue to charge, low and slow, until your battery is fried if you leave it hooked up for extended periods of time. They are not, in any way, what you would call "smart chargers." In fact, they can actually be quite destructive little chargers if they are left hooked up for extended periods of time. Many a good battery has met an early demise while hooked up to one of these chargers without regular monitoring.
I hope this helps to clarify that those little chargers aren't as safe as one would like them to be.
Edited by Euphoria, 18 April 2015 - 09:53 PM.